The Influence of Temperature, Pressure, Salinity, and Surfactant ...

Chem. Eng. Comm., 192: 667–684, 2005 Copyright # Taylor & Francis Inc. ISSN: 0098-6445 print=1563-5201 online DOI: 10.1080/009864490510644

The Influence of Temperature, Pressure, Salinity, and Surfactant Concentration on the Interfacial Tension of the N-Octane-Water System T. AL-SAHHAF Chemical Engineering Department, College of Engineering and Petroleum, Kuwait University, Safat, Kuwait

A. ELKAMEL Chemical Engineering Department, University of Waterloo, Waterloo, Ontario, Canada

A. SUTTAR AHMED Chemical Engineering Department, College of Engineering and Petroleum, Kuwait University, Safat, Kuwait

A. R. KHAN Kuwait Institute of Scientific Research, Safat, Kuwait

An elaborate experimental study was conducted for the determination of the interfacial tension (IFT) values for a n-octane=water system for a wide range of experimental conditions of temperatures, pressures, and salt concentrations that exist in a natural oil reservoir. Three different surfactants were used in the analysis: dodecyl benzene sulfonic acid sodium salt, sodium dioctyl sulfosuccinate, and hexadecyl trimethyl ammonium bromide. The measured IFT values were correlated linearly to pressure (P), temperature (T), and salt concentration (CB) in the aqueous phase. There was an almost 20-fold reduction noticed in the IFT values with the addition of surfactants. This behavior was best related by an exponential expression in terms of the surfactant

Received 14 November 2001; in final form 30 December 2003. Address correspondence to A. Elkamel, Chemical Engineering Department, University of Waterloo, 200 University Ave. West, Waterloo, Ont. N2L 3G1, Canada. E-mail: aelkamel@ cape.uwaterloo.ca

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a0 a1 þ CSa2

þ a3 T þ a4 P þ a5 CB

The experimental data for the three surfactants can be represented independently by the above correlation. The absolute average percentage error was always less than 7% for the different data sets considered. Keywords: Enhanced oil recovery; Interfacial tension; Solubility; Concentration; Regression; n-Octane

Introduction Measurement of the equilibrium interfacial tension (IFT) of hydrocarbon þ water=brine system has many applications in the petroleum, natural gas, and petrochemical industries. For example, in the petroleum industry, the variation of IFT with temperature and pressure strongly influences the transport of the fluid in a reservoir. This makes the IFT the most important factor in keeping approximately 60% of all oil discovered trapped after current oil recovery technologies have been employed (Herd et al., 1992; Gregory, 1994). A number of studies appeared in the past for measuring and predicting the IFT of hydrocarbons against water at reservoir temperatures and pressures (Hassan et al., 1953; Jennings, 1967; Jennings and Newman, 1971; McCaffery, 1972; Matubayasi et al., 1977; Jho et al., 1978; Cai, 1995; Firoozabadi and Ramey, 1988; Nakahara and Arai, 1989; Goebel and Lunkenheimer, 1997; Amin and Smith, 1998; Zuo and Stenby, 1998). Firoozabadi and Ramey (1988), for instance, presented measurements and graphical correlations for estimating the IFT of hydrocarbon liquids against water. Two parameters, density difference and reduced temperature, were used in their correlation. A single curve was found to correlate the hydrocarbon data well. Nakahara and Arai (1989) presented also a correlation (in terms of equations) for predicting the IFT of water-alkane and water-alkylbenzene systems. The validity of their correlation was checked against experimental data from the literature. Recently, Goebel and Lunkenheimer (1997) presented IFT experimental data for the water= n-alkane system and stressed the removal of impurities from the system before measurements are conducted. Amin and Smith (1998) presented IFT measurements for three binary systems (methane-pentane, methane-heptane, and methane-decane). The effect of pressure and temperature on IFT for these systems was studied. Finally, Zuo and Stenby (1998) presented a linear-gradient-theory (LGT) model for computing the interfacial tension of hydrocarbon-water mixtures on the basis of the SRK equation of state assuming that the number densities of each computed component in a mixture are linearly distributed across the interface between the coexisting phases. There are only few studies that dealt with the effect of salt and surfactants in hydrocarbon systems. Ruckenstein and Rao (1987) used a multicomponent isotherm model to calculate the interfacial tension between the decane and brine phases containing a surfactant and a co-surfactant. The model accounts for the different sizes of the solute molecules as well as for the solute-solvent and solute-solute interactions in the surface phase. The interfacial tension was found to decrease with increasing concentration of surfactant. It was found also that at fixed values of surfactant concentration, increasing amounts of co-surfactant and salt have the effect of lowering the

IFT of the N-Octane-Water System

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interfacial tension. Badakhshan and Bakes (1990) studied the effect of different surfactants on IFT for a range of salt concentrations, temperatures, and surfactant concentrations. Three systems were studied: water=n-hexane, water=cycloexane, and water=toluene systems. Cai et al. (1996) presented experimental data on IFT of 10 normal alkane þ water=brine and hydrocarbon mixtures þ water=brine systems. The effects of temperature, pressure, and salt content were studied. It was found that IFT is sensitive to temperature and salt concentration but weekly dependent on pressure. Liggieri et al. (1995) performed measurements of IFT of a hexane=water plus Triton X-100 systems. From the interpretation of their data, their main conclusion was that the Freundlich isotherm describes the system well in the range of concentrations they considered. In the present study an extensive program was established to measure the IFT of the n-octane=water system under a wide range of conditions of temperatures, pressures, salt, and surfactant concentrations. Three different surfactants were screened: two cationic and one anionic. The video pendant drop method was used for the measurement of IFT. The experimental data was analyzed and tested for various existing correlations in the literature. A generalized equation has been developed to express IFT as a function of surfactant concentration, salt concentration, temperature, and pressure for each surfactant. To validate the suggested correlation, a statistical test was performed to establish the percent average absolute error between the predicted and measured values of IFT.

Experimental Work Material and Equipment The hydrocarbon n-ocatne (reagent grade, 99% GC pure) was purchased from Fluka Chemicals Co. and double distilled, deionized water was obtained from the analytical laboratory facilities in the Chemical Engineering Department at Kuwait University. Three surfactants, dodecyl benzene sulfonic acid sodium salt, sodium dioctyl sulfosuccinate (Alcopol O 70 PG), and hexadecyl trimethyl ammonium bromide (Cetrimide), of commercial grade were purchased from Fluka, Aldrich, and Sigma Chemicals Co., respectively. A pendant drop apparatus enhanced by video imaging was employed for the measurements of interfacial tension (Satherley et al., 1990; Guo and Schechter, 1997). The apparatus permits the accurate measurement of IFT values as low as 1 mNm 1 under high temperatures and pressures. The IFT cell of model IFT-10 serial number 3381 operates up to a pressure of 69 MPa (10,000 psig) and a temperature of 450 K (350F) covering the entire range of pressures and temperatures existing in most reservoirs on the globe. The IFT cell can be heated by a heating jacket to achieve the required temperature. The cell must be leveled on top of a vibration-free table for accurate measurements of interfacial tension. The IFT cell has a sensitive metering valve with a vernier for appropriate control of the organic phase (oil) drop formation. The other valves serve to isolate the system during needle tip change to allow introduction of the different fluids. The stainless steel needle tip and holder are sealed with a Teflon gasket. The back-pressure regulator plumbed to the top of the cell allows for flushing out. The trinocular microscope can be used to view the formation of the oil droplet and to measure droplet dimension photographically (Polaroid attachment or video camera system). A schematic of the apparatus is shown in Figure 1.

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Figure 1. Experimental apparatus used for measuring IFT.

Procedure The internal valves and vessels were flushed with chromic acid, steam, and then distilled water. The densities of aqueous and organic (oil) phases at required temperatures were measured, and the density difference between these two liquids was calculated. The drop-forming pumps and vessels were filled with distilled water and organic phase (n-octane). The pressure pumps and temperature device were calibrated and the IFT cell was operated at the specified temperature and pressure conditions. The surfactant was dissolved in the aqueous solution and placed in one side of the vessel, while n-octane on the other side attached with pressure pumps. The surfactant solution and n-octane were injected into the IFT cell at the required temperature and pressure. The oil droplet was filmed by a video camera after equilibrium was reached. Equilibrium was ensured by monitoring the IFT as a function of time up to the point when no further change was detected. Measurement of the oil drop dimensions and fluid densities enabled the calculation of interfacial tension, IFT. By using the pendant drop cell, the IFT can be calculated by the following equation: r¼

Dqgde2 H

ð1Þ


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Figure 2. Oil droplet dimensions for the calculation of IFT.

where Dq is density difference between the two fluids in g cm 3, de is maximum diameter of the real (not magnified) drop in cm, g is gravitational constant at the point of measurement in cm s 2, 1=H is shape factor based on ds=de ¼ fðsÞ, do is diameter of the tip of the actual needle in cm, and dn is diameter of the needle after magnification in cm. These variables are explained in the sketch of a pendant drop given in Figure 2. The magnification factor was evaluated by the ratio of actual nozzle diameter, dn, to the needle diameter on video film, do (dn=do). This scale is used to provide precise values of de and ds. The value of 1=H was obtained from the table provided by Bartell and Niederhauser (1949) as a function of the ratio (ds=de). In order to validate the experimental procedure outlined above, each measurement was repeated at least three times for the specified temperature and pressure. The reproducibility was excellent within an acceptable experimental limit of error of less than 0.25%. In addition, the IFT values at different pressures and at a temperature of 25C that were reported by Cai et al. (1996) were compared to the measurements of this work (Table I). As can be seen, the agreement is consistent with a percent deviation error always less than 0.4%.

Results and Discussion The experimental program for studying the three synthetic surfactants (dodecyl benzene sulfonic acid sodium salt, sodium dioctyl sulfosuccinate, and hexadecyl trimethyl ammonium bromide) is shown in Table II. Experiments were conducted with both brine and distilled water. The effect of temperature, pressure, salt, and

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Table I Comparison of measured IFTs of this work and those reported by Cai et al. (1996) for the n-octane=water system Presure (Mpa)

Temperature (C)

Measured IFT (mN=m) this work

Cai et al. IFT (mN=m)

% Deviation

25 25 25 25 25 25 25 25 25 25 25 25 25 25 25

51.29 51.39 51.41 51.52 51.64 51.66 51.76 51.83 51.89 52.01 52.04 52.13 52.26 52.29 52.38

50.92 51.05 51.07 51.20 51.36 51.38 51.51 51.59 51.67 51.82 51.86 51.98 52.13 52.17 52.29

0.370 0.343 0.335 0.315 0.282 0.276 0.246 0.238 0.219 0.192 0.182 0.151 0.126 0.119 0.091

0.69 3.45 4.04 6.90 10.35 10.91 13.8 15.54 17.25 20.7 21.54 24.15 27.6 28.42 31.05

surfactant concentration was studied by appropriately varying the ranges of these variables. Effect of Surfactants The three surfactants with varying concentrations were used in the aqueous phase, and IFT was determined for n-octane at different temperatures and pressures. The concentration of each surfactant was gradually increased, and IFT was measured until the turbidity in aqueous phase made it impossible to form a stable drop for the determination of IFT. Table II Details of the experimental work conducted in this study Name of surfactant

Temperature range (C)

n-Octane=water system Surfactant A 25–65 Surfactant B 25–65 Surfactant C 25–65 n-Octane=brine system Surfactant A 25–65 Surfactant B 25–65 Surfactant C 25–65

Pressure Surfactant Salt range concentration concentration No. of (MPa) range (wt%) range (wt%) experiments 3.45–31.05 0.69–31.05 3.45–31.05 3.45–31.05

0–10 0.01–2 0–11

0 0 0

275 200 120

3.45–31.05 3.45–31.05 3.45–31.05

1–10 0.01–2 0–11

1–3.12 1–2.5 1–2.5

1375 800 600


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Dodecyl Benzene Sulfonic Acid Sodium Salt. Eleven different concentrations ranging from 0 to 10 wt% were tested with dodecyl benzene sulfonic acid sodium salt (surfactant A) for five temperatures and five different pressures, resulting in a total of 275 values of IFT. In Figure 3 the IFT values for n-octane are shown as a function of the concentration of surfactant A at a fixed pressure of 3.45 MPa (500 psi) and at three different temperatures. The IFT values undergo an exponential decrease with increasing surfactant concentration. Badakhshan and Bakes (1990) have, however, described this functionality as linear due to a very narrow range of surfactant concentration used in their experimental program. The dependency on temperature is linear (Figure 4) in accordance with what was reported by Badakhshan and Bakes (1990). The effect of pressure for four selected surfactant concentrations is given in Figure 5. As can be seen, the IFT values are directly proportional to pressure.

Sodium Dioctyl Sulfosuccinate (Alcopol O 70 PG). Four different concentrations of sodium dioctyl sulfosuccinate (surfactant B) ranging from 0 to 2 wt% were tested for five temperatures and 10 different pressures, resulting in a total of 200 values of IFT. The solubility of surfactant B has been limited. The maximum workable concentration could not be more than 2 wt%. A concentration greater than 2 wt% produces turbidity. In Figure 6, the IFT values for n-octane are plotted as a function of concentrations of surfactant B at a fixed pressure of 17.25 MPa (2500 psig) and at three different temperatures. The IFT values are also exponentially related to the surfactant concentration. There is a major reduction of about 25-fold in the value of IFT at 2 wt% concentration of surfactant B. The relationship with temperature is linear (Figure 7). The influence of pressure is shown in Figure 8. As can be seen, the dependence on pressure is also linear.

Figure 3. IFT of the n-octane=water system as a function of concentration of surfactant A at a fixed pressure of 500 psi and at three different temperatures. The solid lines represent Equation (2).

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Figure 4. IFT of the n-octane=water system as a function of temperature at a fixed pressure of 500 psi and at four different concentrations of surfactant A. The solid lines represent Equation (2).

Hexadecyl Trimethyl Ammonium Bromide (Cetrimide). Six different concentrations of hexadecyl trimethyl ammonium bromide (surfactant C) ranging from 0 to 11 wt% were tested for five temperatures and four different pressures, resulting in a total of 120 values of IFT. The IFT values for the n-octane=water system are plotted

Figure 5. IFT of the n-octane=water system as a function of pressure at a fixed temperature of 25C and at four concentrations of surfactant A. The solid lines represent Equation (2).


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Figure 6. IFT of the n-octane=water system as a function of concentration of surfactant B at a fixed pressure of 2500 psi and at three different temperatures. The solid lines represent Equation (2).

as a function of concentrations of surfactant C at a fixed pressure of 31.05 MPa (4500 psi) and at three different temperatures (Figure 9). The influence of temperature and pressure are shown in Figures 10 and 11, respectively. Comparing Figures 3, 6, and 9, it is easy to see that surfactant C lowers the IFT at the n-octane=water system the most. Surfactant C is, however, more costly than surfactants A and B (0.783 KD=g compared to 0.153 KD=g and 0.583 KD=g).

Figure 7. IFT of the n-octane=water system as a function of temperature at a fixed pressure of 500 psi and at three different concentrations of surfactant B. The solid lines represent Equation (2).

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Figure 8. IFT of the n-octane=water system as a function of pressure at a fixed temperature of 25C and at three different concentrations of surfactant B. The solid lines represent Equation (2).

Effect of Salt Most oil formations have high salinity, the effect of which on the IFT values in oil=brine systems is complex. Ruckenstein and Rao (1987) have discussed this behavior for oil=brine=surfactant and co-surfactant systems. They reported that

Figure 9. IFT of the n-octane=water system as a function of concentration of surfactant C at a fixed pressure of 4500 psi and at three different temperatures. The solid lines represent Equation (2).


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Figure 10. IFT of the n-octane=water system as a function of temperature at a fixed pressure of 500 psi and at four different concentrations of surfactant C. The solid lines represent Equation (2).

an oil-in-water micro-emulsion coexists with excess oil at a low concentration of salt, resulting in a decrease in the IFT values with an increase in salinity. At high salt concentrations a water-in-oil micro-emulsion coexists with excess water, and the IFT values between the micro-emulsion and excess water increases with an increase

Figure 11. IFT of the n-octane=water system as a function of pressure at a fixed temperature of 25C and at four different concentrations of surfactant C. The solid lines represent Equation (2).

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in salinity. Cai et al. (1996) have measured the IFT values for normal alkanes with water and brine. They concluded that the IFT values are linearly proportional to the pressure. They have also noted that while the salt concentration increases the IFT value, the salt type does not have any noticeable effect. Because of such conflicting results in the literature by different researchers, the present article will investigate thoroughly the influence of various salt concentrations on IFT values as a function of pressure and temperature and with varying concentrations of the three different surfactants. Eleven different concentrations ranging from 0 to 10 wt% of surfactant A were tested for five temperatures and five pressures (500, 1500, 2500, 3500, and 4500 psi), and five different salt concentrations ranging from 0 to 3.12 wt% were considered. Figure 12 shows the variation of IFT as a function of different salt concentrations at a fixed pressure, a fixed temperature, and for four different concentrations of surfactant (A). The IFT values undergo a decrease with an increase in salt concentration in the aqueous phase. The presence of salt alters the distribution of surfactant between the oil and aqueous phases. The activity coefficient of the salt increases and the salt molecules transfer to the oil phase. Even though at low salt concentrations, the surfactant is concentrated in the aqueous phase, it is gradually transferred to the interface, leading to a decrease in IFT values. This result is in contrast to the conclusion drawn by Cai et al. (1996), who observed an increase in the value of IFT by introducing salt into the aqueous phase. Four salt concentrations, ranging between 0.01 and 2.5 wt%, were tested with varying surfactant B concentrations (up to 2 wt%) for five temperatures (25, 35, 45, 55, and 65C) and 10 different pressures. The influence of salt concentration on IFT is depicted in Figure 13.

Figure 12. IFT of the n-octane=water system as a function of concentration of salt at 25C, 500 psi, and at four different concentrations of surfactant A. The solid lines represent Equation (2).


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Figure 13. IFT of the n-octane=water system as a function of concentration of salt at 25C, 4500 psi, and four different concentrations of surfactant B. The solid lines represent Equation (2).

For surfactant C, five concentrations of salt were tested with six surfactant concentrations (from 0 to 11 wt%), five temperatures (25, 35, 45, 55, and 65C), and four different pressures. The IFT values are plotted in Figure 14 at 25C and at a fixed pressure of 3.45 MPa (500 psi). The same general trend is observed as with the previous two surfactants. Ruckenstien and Rao (1987) noted that for high salt concentrations, the coexistence of water-in-oil micro-emulsion with excess of water results in an increase in IFT values with an increase in salinity. In the present analysis we have tested up to 2.5 wt% salt concentration, and a continuous decrease was observed with an increase in salt concentration. The IFT values showed a linear dependence on pressure and temperature and an exponential dependence on surfactant concentration. In the next section, an attempt will be made to prepare an empirical model based on the collected data to capture the variation of IFT with the different variables considered.

IFT Empirical Modeling In a given system where several variables change, it is of interest to know and quantify the effect that some variables exert on others. The functional relationship between the variables could be a simple relation, as most commonly assumed for physical processes. Often the functional relationship is very complicated to describe by simple terms or functions. In such situations the functional relationship is

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Figure 14. IFT of the n-octane=water system as a function of concentration of salt at 25C, 500 psi, and at four different concentrations of surfactant C. The solid lines represent Equation (2).

approximated by a simple mathematical function like inverse, polynomial, or an exponential of a certain degree. The approximation function is mostly constrained to a limited range of variables according to the available data representing the behavior of the complicated relation (Chapra and Canale, 1998; Draper and Smith, 1981). Regression analysis provides an estimate of parameters of the approximate functional relationship. By examining the approximate function we may learn more about the true relation between the variables and appreciate the separate and joint effects produced by changes in certain important variables. Improvements in the functional relationship to include some of the physical behavior of the variables give a more acceptable relation. In the present study, the IFT values are a function of surfactant concentration CS, salt concentration CB, operating temperature T, and pressure P. It is obvious from Figures 4, 5, 7, 8, 10, 11, 12, 13, and 14 that the IFT values are linearly related to temperature, pressure, and salt concentration. The influence of surfactant concentration is not linear (Figures 3, 6, and 9) and can be best represented by an exponential form. Various forms have been tried and the best equation that can represent the entire data within an acceptable range of average error was found to be of the form: IFT ¼ Exp

a0 a1 þ CaS2

þ a3 T þ a4 P þ a5 CB

ð2Þ

To quantify the degree of accuracy of the approximate functional relation and compare it to the true relation between the variables, a number of statistical tests were performed. These tests include the average error, the standard deviation,


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and the correlation coefficient. In addition, we considered the variables in different sets at a time (i.e., variation with temperature and pressure only, variation with temperature, pressure, and salt concentrations, and variation with respect to all four variables: temperature, pressure, salt, and surfactant concentration). The experimental data for each surfactant have been first analyzed to assess the dependency on temperature and pressure only (i.e., a1 ¼ a2 ¼ a5 ¼ 0). The results are shown in Table III with an average absolute error for each surfactant concentration. The coefficients of pressure and temperature terms are not constant, indicating that the behavior of the aqueous phase (surfactant solution) has completely changed with the alteration of surfactant strength. Similarly, the dependency on temperature, pressure, and salt concentration (i.e., a1 ¼ a2 ¼ 0) is shown in Table IV. The generalized form of the correlation (Equation (2)) has been tried for the entire data for each surfactant. The coefficients of the correlation are listed in Table V along with the average absolute percentage error for the three surfactants. We note finally that the solid lines in Figures 3–14 represent the developed correlation for the different experimental conditions. As can be seen, the fit is good.

Table III Testing the linear dependence of IFT on temperature and pressure for the three surfactant’s concentrations when no salt is present Surf. Conc. Wt%

Exp(a0)

a3

a4 10 4

Error %

Surfactant A 0 1 2 3 4 5 6 7 8 9 10

54.27 34.05 21.48 14.64 9.88 10.05 6.509 6.477 5.391 5.233 4.604

0.1210 0.1530 0.0867 0.0656 0.0432 0.0462 0.0355 0.0500 0.0405 0.0485 0.0478

2.586 0.0896 0.9998 7.050 11.60 1.654 5.670 4.867 3.640 3.402 3.543

0.548 3.44 2.55 3.41 2.55 4.74 1.33 3.51 5.55 5.23 3.09

Surfactant B 0 0.01 1 1.5 2

3.998 3.782 2.023 1.993 1.683

0.0024 0.0004 0.0007 0.0076 0.0046

Surfactant C 0 1 5 6 10 11

54.24 4.104 1.503 1.409 1.277 1.227

0.1207 0.0159 0.0056 0.0055 0.0059 0.0059

4.430 15.50 78.20 104.00 97.10 2.612 2.041 0.8206 0.7746 0.6069 0.7302

0.556 0.603 2.06 1.61 4.73 0.173 1.89 3.63 2.97 4.94 4.38

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Table IV Testing the linear dependence of IFT on temperature, pressure, and salt concentration at different surfactant concentrations Surfactant wt%

Exp(a0)

a3

a4 10 4

a5

Error %

Surfactant A 0 1 2 3 4 8 10

52.48 28.45 17.28 12.85 10.53 5.233 3.075

0.0815 0.0275 0.0223 0.0413 0.0457 0.0394 0.0197

7.224 5.766 5.389 5.427 4.417 5.225 3.875

2.083 0.1962 0.0314 0.0769 0.0895 0.0060 0.7171

0.470 1.51 2.62 2.48 3.02 3.56 3.61

Surfactant B 0 0.01 1 1.5 2

48.77 18.03 7.276 7.116 6.231

0.0001 0.0616 0.0486 0.0503 0.0499

7.562 9.280 7.847 6.666 6.233

2.074 1.512 0.3352 0.1101 0.1093

2.39 1.72 4.01 2.40 3.32

Surfactant C 0 1 5 10 11

43.31 3.012 2.000 1.426 1.350

0.0309 0.0035 0.0018 0.0033 0.0010

5.884 3.502 0.0178 0.1200 0.0252

0.1693 0.1705 0.0191 0.0199 0.0202

7.22 4.13 4.71 4.32 5.26

Table V Constants in equation (2) for the octane=water and octane=brine systems Name of surfactant

System

a0

a1

a2

Surfactant A Octane=water 24.94 6.354 0.9879 Surfactant B Octane=water 4.859 1.249 0.7629 Surfactant C Octane=water 2.163 0.554 0.9420 Name of surfactant

System

a0

a1

a2

a3

a3

a4 10 4

Error %

0.0516 0.0303 0.0060

3.933 6.873 0.740

4.82 2.27 3.87

a4 10 4

a5

Surfactant A Octane= 25.92 6.825 1.1337 0.0151 5.248 0.2807 brine Surfactant B Octane= 6.185 1.5880 0.1310 0.0542 6.600 1.425 brine Surfactant C Octane= 1.764 0.4645 0.6035 0.0024 0.6005 0.2101 brine

Error % 5.32 3.49 6.54


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Conclusion In this article, experimental values of IFT for the n-octane=water system were reported. The effects of temperature, pressure, salt, and surfactant concentrations were studied. Three different surfactants were screened to determine the best surfactant that can lower the IFT values for facilitating enhanced oil recovery (EOR). The measured IFT values were found to be linearly dependent on pressure and temperature, confirming previously, reported results in the literature. There was an almost 20-fold reduction noticed in the IFT values with the addition of surfactants. This behavior was best related by an exponential expression. The suitability of the three surfactants was checked by cost, solubility, and effectiveness in reducing the IFT values in different operating temperatures and pressures. The effect of salt was studied using five different concentrations (up to 3.12 wt%) and the most appropriate expression relating the salt concentration and other variables with the IFT values was determined. The dependence of the IFT on salt concentration was found to be linear within the experimental range considered. Further work is in progress to use crude oil and formation water in the presence of different divalent ionic salts that coexist in the local rock formations in oil reservoirs. Selection of the correct type of surfactant that could dislodge oil from depleted oil wells is the main objective of this research for EOR. The present work sets up the basis for further research.

Nomenclature Cs CB P T a0 a1 a2 a3 a4 a5 de ds g 1 H

do dn Dq IFT

surfactant concentration (wt%) Salt concentration (wt%) Pressure (MPa) Temperature (K) Constant Constant Power of surfactant concentration Temperature coefficient Pressure coefficient Salt concentration coefficient Maximum diameter of the real (not magnified) drop in cm Selected diameter of the real (not magnified) drop in cm Gravitational constant at the point of measurement in cm S 2 Shape factor based on the ratio of ds and de Diameter of the tip of actual needle in cm Diameter of the needle after magnification in cm Density difference between the two fluids in gcm 3 Interdfacial Tension (mN=m)

Acknowledgment The authors would like to thank the research administration at Kuwait University for partially sponsoring this research under grant EC072. We would also like to thank Prof. El-Gibaly for many helpful discussions at the beginning of this work.

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